The present invention relates generally to rotary electric machines, for motor operation as well as for generator operation, and specifically relates to a method of optimizing the performance utilization of electric machines, a rotary electric machine, a use of such a machine and a rotor for such a rotary electric machine.
The development within this area is presently directed towards an ever higher output in terms of power and torque, for given machine sizes. This means that manufacturers of such machines are pressed, primarily by the current tough price competition, to increase the performance of the machines so that smaller machines that are less expensive to manufacture may be employed for a given range of power output.
In conventionally designed machines such a development towards electric machines having higher and higher performance utilization, that is an increased output of power and torque, has led to increased dimensions for the rotor winding conductors and to the coherent increased dimensions for the rotor winding grooves, especially with reference to the radial depth of the grooves. To a great extent this depends upon the fact that in such machines it has not been possible to obtain a cooling that is so efficient that the size of the rotor winding grooves can be kept down.
To exemplify this it can be mentioned that in direct current machines the conventional cooling is based on axial cooling channels 5, 6 provided in accordance with a standardized hole configuration in the rotor 1, i.a. such as it is illustrated in the enclosed FIG. 1. In the normal case the same hole configuration is employed for all of the different numbers of winding grooves 4 used for a given rotor diameter in a particular series of motors. Thereby the number of cooling holes or channels being positioned at the same radial distance from the centre C of the shaft is normally an integer multiple of the pole number of the machine, in order to provide a relatively constant summated flow in the magnetic circuit through the rotor, irrespective of the polar position of the rotor. Expressed in an other way, the number of outer cooling channels as well as the number of inner cooling channels are each an integer multiple of the pole number. Since the number of winding grooves varies and is normally not divisible with the pole number, the distances between the rotor winding grooves and the rotor cooling channels will not be the same anywhere around the circumference of the rotor. This in turn leads to the fact that the magnetic circuit will have an unequal flow distribution at the respective winding grooves around the circumference of the rotor. In order to minimize the negative effects thereof upon the electrical properties of the machine, the distance between the winding grooves and the cooling channels must be designed relatively large, with the accompanying poor cooling and greatly limited power and torque output.
In order to increase the power and torque output from such a direct current machine having a conventional cooling hole configuration in the rotor, in accordance with the above, the rotor conductor area and thereby the dimensions of the winding grooves must be increased, as stated above. Simultaneously the distances between the winding grooves and the cooling channels must be made smaller. However, due to the above described unequal flow distribution, said measures impair the electric properties of the machine, which in particular results in a considerably decreased commutation capacity at high power outputs. Thereby, it is not unusual for rotary electric direct current machines having a conventional cooling, to be impaired by such substantially increased electrical strain or stress that the contact function between the brush and commutator reaches an upper limit. In the absence of any margin for this contact function, the direct current motor becomes very sensitive to external disturbance.
To sum up, it may therefore be established that the conventional design in itself has performed well, with the above discussed limitations. However, it is based on the unfortunate compromise between on the one hand a desire to achieve an efficient cooling and, on the other hand the aim towards providing a machine having good electrical properties.
In the light of the above discussion, a basic object of the invention is to provide a simple method of combining high performance in a rotary electric machine with an improvement of the electrical properties thereof. Expressed otherwise, the aim is to eliminate the need to compromise between efficient cooling and good electrical properties. To be precise, this object is obtained specifically for a direct current machine by providing wide black bands, that is low commutation strain for the electric machine in order to thereby obtain a functionally stable operation with low maintenance.
The invention is based on the understanding that the need for said compromise can be eliminated and that, instead, the cooling and the electrical properties can both be optimized simultaneously by creating symmetry in the magnetic circuit of the motor. According to the invention this is achieved by providing at least the mainly effective, outer axial cooling channels in the rotor of the machine essentially symmetrically with reference to the magnetic flow paths in the rotor. Thereby a symmetrical design of the magnetic circuit of the machine is obtained, whereby a good electromechanical motor function is obtained with, for a direct current machine, low maintenance of brushes and commutator. Simultaneously wide shunt regions can be obtained without reduction of the rated output.
In one embodiment of the invention the symmetrical design of the magnetic circuit, according to the basic object of the invention, is combined with the provision of a great number of outer cooling channels in the rotor, positioned near the rotor winding grooves, that is at a large radial distance from the centre of the rotor shaft. By combining the symmetry of the magnetic circuit and the large number of outer cooling channels a very good possibility is provided for improving the performance utilization of a rotary electric machine. In particular this improvement is achieved by the fact that the good electrical properties provided in accordance with the basic object of the invention can be maintained and even improved further by means of the increased cooling capacity that in turn permits the reduction of the size of the winding grooves. Through the symmetrical positioning of the cooling channels they can be provided in a large number and at the same time they can be moved up close to the winding grooves and thereby close to the source of the power loss in the form of resistive loss in rotor conductors in rotor winding grooves and magnetic or iron loss in rotor groove teeth.
In accordance with further preferred embodiments of the invention the outer cooling channels are provided in a number corresponding to half of the number of rotor winding grooves when the latter is even, or alternatively in a number that is an integer multiple of the number of rotor winding grooves.
In accordance with a further embodiment the cooling channels are provided with an enlarged inner peripheral surface and/or cross section area. This is obtained partly by the positioning of the cooling channels further out from the centre of the shaft of the machine, which allows for cooling channels having a larger cross section area, and partly by giving the cooling channels an elongated form, as seen in the radial direction, and/or forming them with a profiled, for instance polygonal or wavy surface.
Hereby a further improved cooling capacity is obtained, which permits raising the current density in the rotor winding. This in turn contributes further to the reduction of the dimensions of the rotor winding grooves. For a direct current machine this means that the commutation strain can be reduced.
Other advantages that are obtained by means of the invention are that the temperature gradient radially through the rotor plate can be reduced. This means that, compared to plates having a conventional cooling configuration, larger rotor plates can be cold-pressed on the rotor shaft since the shaft hole of the rotor plate does not expand so much in the operating temperature condition, that the grip between the shaft and the rotor plate is lost. In these embodiments the cooling surfaces are increased and provided closer to the heat source/power loss source, which means that temperature peaks in the winding grooves can be reduced at temporary overloads.
According to another aspect of the invention a rotor for a rotary electric machine is provided, which employs the principles of the present invention.
A further aspect of the invention relates to the use of a rotary electric machine designed in accordance with the basic principles of the invention, for motor operation.
Further objects, features and advantages of the invention are indicated in the dependent claims and in the following description of exemplifying embodiments thereof.